Mitochondria produce the bulk of cellular energy and thus are essential to normal physiological function. Defects in mitochondria! maintenance and activity have been widely implicated in human disease including myopathy, neurodegenerative disease, diabetes and ageing. Although many pathological mutations are localized to mitochondrial DNA, nuclear genes encode the vast majority of products required for mitochondrial functions, including the maintenance and expression of the mitochondrial genetic system. This is a proposal to continue studies on the molecular mechanisms governing nucleo-mitochondrial interactions in mammalian cells.The specific aims will focus on transcriptional activators and coactivators that integrate the expression of the respiratory apparatus with cell growth. Our previous studies led to the discovery, purification and molecular cloning of nuclear respiratory factors (NRFs). These transcription factors act on the majority of nuclear genes whose products are required for mitochondrial biogenesis and function and include respiratory subunits, heme biosynthetic enzymes, protein import and assembly factors, and key components of the mitochondrial transcription and replication machinery. In recent years, NRFs have been identified as important targets for the PGC-1 family of transcriptional coactivators. These coactivators are induced by physiological signals and have the ability to integrate the actions of NRFs and other transcription factors in orchestrating programs of gene expression essential to cellular energetics. One PGC-1 family member, which we have designated PGC-1-related coactivator (PRC), is induced during proliferative growth and may play an important role in integrating the cell growth program with the expression of the respiratory apparatus. PRC is indistinguighable from other PGC-1 family members in its ability to interact with NRFs and to direct the expression of NRF target genes. The specific aims of this proposal are directed at understanding PRC molecular interactions and biological functions as follows: 1) Develop cellular systems for investigating PRC in vivo function. 2) Establish the specific contribution of PRC to mitochondrial expression and function. 3) Define the spectrum of PRC-regulated genes. 4) Investigate the molecular basis for PRC transcriptional and post-transcriptional expression. 5) Characterize key molecular interactions related to PRC function.